In telecommunication systems, customers are provided with different classes of service depending on their particular needs. Certain customers such as the police and fire departments are provided with so-called "essential lines" which, among other things, assure these customers of access to the network during overload periods.
While most switching offices in a telecommunication network are designed to handle the expected traffic load during the busiest hours, circumstances such as bad snow storms, hurricanes, local catastrophes, etc., may overload the system beyond the load expected during the normal busy period. Under such overload conditions, service may be degraded to the point where delays in getting dial tone are experienced by the customers.
In modern electronic switching systems, lines initiating a service request are first put in a queue waiting to be served by the common control equipment. When the line is ready to be served, the common control equipment removes the line from the queue and connects the line to a digit receiver. The digit receiver transmits dial tone over the calling line to the customer station and is equipped to store the digits transmitted from the customer station. The time interval between the initiation of a service request by a customer and the provision of dial tone is generally used as a measure of the load on a system. Thus, overloads are sometimes defined in the terms of the number of consecutive calls experiencing long dial tone delays recorded at a particular time in the switching system.
In certain prior art switching systems, a first-in/first-out (FIFO) queue was used to store the identity of lines waiting for service. With a FIFO queue, the line waiting the longest was served first and everyone was more or less equally delayed in getting dial tone. In one known system using a FIFO queue, a line load control machine mechanism was actuated when a predetermined number of consecutive long dial tone delays was experienced.
In this system, the line load control was implemented by a technique employing a rotating mask which limited the number of lines that could originate calls. As the load increased, more nonessential lines were masked or blocked until a limit was reached whereby only essential lines were being served.
While this arrangement was suitable for its intended purpose, it was not without problems. For example, when the mask was rotated those lines previously blocked would exhibit a significant increase in the number of ineffective attempts due to the delay in dial tone they experienced. Also, the call processor was subject to an oscillating load condition. More specifically, when the rotating mask was in effect, the overload condition which initiated the line load control was suppressed. This led to a false termination of a line load control and removal of the mask only to have the overload condition reappear.
In the alternative to the FIFO queue, some switching systems use a last-in/first-out (LIFO) queue for processing service requests. With the LIFO queue, the last line requesting service is served first, and older service requests are pushed down in the queue and may experience longer delays.
It was found in these systems using a LIFO queue, that the line load control mechanism was hardly ever actuated since the control mechanism was triggered only after a predetermined number of consecutive dial tone delays was detected, and this was unlikely to occur using the LIFO queue. Accordingly, the line load control was virtually ineffective and essential lines were denied preferential treatment during overloads.